Community Climate System Model (CCSM)
Project Goal
The primary goal of the Community Climate System Model (CCSM) project is to develop a state-of-the-art climate model and to use it to perform the best possible science to understand climate variability and global change. We will strive to build a CCSM community of users who are interested in participating in this project.
CCSM Science Plan and Strategic Business Plan
The Community Climate System Model (CCSM) project office produced two documents during this past year. The CCSM Science Plan 2004-2008 was approved by the Scientific Steering Committee and released in June 2003. It describes a program for the mitigation of CCSM biases, as well as applications of the CCSM to science questions of concern to the Intergovernmental Panel on Climate Change (IPCC) and of the U.S. Climate Change Science Program. The plan also outlines new directions for the development of the CCSM to include much more comprehensive treatment of the biogeochemical processes that operate to determine the chemical and physical characteristics of the climate system and to make the CCSM more broadly applicable to the needs of decision makers.
The project office also produced a CCSM Strategic Business Plan that outlines the staffing needs of scientists and software engineers and computer resource requirements necessary to execute the CCSM Science Plan.
The major preoccupation of the CCSM project, as a whole, was to diagnose and decrease the biases that were evident in the long-term integration of CCSM2. The objective was to produce a version of CCSM that had the highest fidelity possible for use in the Fourth Assessment Report (FAR) of the IPCC. This objective involved a reconsideration of the parameterization of cloud processes in the atmospheric component and adjustments in the land surface model in respect to the treatment of snow covered surfaces. As is the case in a coupled model, such changes in the parameterizations of one component had effects on the behavior of the other components. There were also adjustments to the ocean model and the sea ice model to achieve balanced and much less biased simulations of the properties of the climate system for present forcing conditions. In addition, the project has produced a version of CCSM that has much higher resolution in the atmospheric component, T85 as compared to T42, while holding to the same physical parameterizations in the two versions. This will enable a systematic exploration of the effects of resolution on climate simulations and climate sensitivity, provide better measures of the uncertainty associated with simulations of global warming, and give more detail on the potential changes for the assessment community.
Atmospheric Component
The Atmosphere Model Working Group (AMWG) has undertaken a number of developments to address model biases, improve performance and portability, improve the physical representation of various processes, introduce representations for missing physical processes, and provide more flexibility in modes of interaction with other components of CCSM. Many of these modifications were discussed in presentations at the June 2003 annual CCSM workshop in Breckenridge, Colorado. We refer to the model outlined at that workshop as the Community Atmosphere Model (CAM2.X). It is anticipated that it will be released as CAM3 near the end of the calendar year 2003.
These developments had two motivations:
1) to continue the normal evolutionary process pursued by members of the AMWG to improve the representation of atmospheric processes and reduce model biases; and
2) to accelerate the effort taking place to prepare the model for use in the upcoming IPCC assessment activity.
Improvements to the physics include:
1) A substantially revised prognostic cloud water parameterization that includes separate phases for ice and liquid condensate, advection and sedimentation of condensate, and a consistent treatment of condensate in the microphysics and radiative transfer parameterizations. The latent heat of fusion is now included in all aspects of the thermodynamics, involving the phase transformation of water substances. The model now conserves energy exactly in all physical parameterizations. The status of both phases of water is now communicated to other components of the climate system. The shallow/frontal convective parameterization now interacts more closely with the prognostic cloud parameterization by detraining condensate directly into the stratiform clouds.
2) A significant improvement was made to the representation of direct short-wave aerosol forcing. An aerosol distribution for sulfate, dust, sea salt, and carbonaceous aerosols is now included in the model. This distribution has an annual variation, but it is the same from year-to-year. There is also an optional package for the prognostic representation of these aerosols.
3) The long- and short-wave radiative transfer parameterizations have been revised to include more recent characterizations of water vapor absorption and aerosol scattering and absorption (in the short-wave) and absorption and emission by greenhouse gases (in the long-wave).
These modifications have been evaluated in stand-alone CAM runs and included in coupled CCSM runs with newer versions of the other components of the climate system. The model simulations represent a substantial improvement over CAM2. In particular, the warm bias present in the CAM2 arctic simulations has been remedied, the cold bias in the tropical tropopause temperature has been significantly reduced, and the cloud response to tropical sea surface temperature (SST) variations is now significantly more realistic. Each of these improvements address deficiencies identified by the AMWG at the 2002 annual CCSM workshop as a problem requiring attention by the CAM community. We anticipate the release of a complete set of model source code, documentation, initial and boundary datasets, and control integrations at a variety of horizontal resolutions, complete with diagnostic analyses to the community via the Web in late 2003 or early 2004. The code has been tested on a variety of different computer architectures, and significant improvements have been made to performance and portability. One of the configurations of the model released at that time will match that to be used for the simulations for the upcoming IPCC effort. The distribution will also contain configurations useful for runs at other resolutions with a variety of dynamical "cores."
Land Component
The Land Model Working Group (LMWG) undertook several projects to reduce prominent biases in the Community Land Model (CLM2). A new under-canopy turbulence scheme was adopted to reduce the excessively warm daytime ground temperatures in sparsely vegetated areas. A new parameterization of fractional snow cover on the ground was proposed to improve the low fractional snow cover in CLM even with deep snow packs. The proposed parameterization uses different relationships between snow depth and fractional snow cover during the snow accumulation and snow melt phases. The transition between these two phases was problematic and only the accumulation phase was accepted for implementation in CLM. This new parameterization increased the fractional snow cover on the ground and therefore increased surface albedo in the arctic during winter. This cooled surface temperature and helped eliminate a prominent high-latitude winter warm bias in CCSM2. However, the parameterization was not formally adopted for the next version of CCSM because the surface cooling led to excessive sea ice in the arctic.
Active research was undertaken to understand and improve other known biases in the model related to high evaporation of intercepted water. Downscaling of rainfall within a grid cell is thought to be key to improving the interception of rainfall. The implementation of sunlit and shaded leaves in CLM2 is also deficient. Changes to this and to the Vmax parameter that controls photosynthesis and stomatal conductance are needed to improve the simulation of gross primary production and also alleviate some of the low transpiration bias in CLM2. Runoff generation based on a topographic index was also advocated, but not yet adopted for CLM.
New capabilities being developed for CLM include biogeochemistry (carbon and nitrogen cycles, mineral aerosols [see Biogeochemistry Working Group section]), dynamic vegetation, prognostic canopy air space, water isotopes, and land cover and land use change, including an urban land cover parameterization. During the last half of the year, much time was spent developing a vector version of CLM. When finished, this will provide a single code for scalar and vector platforms, will maintain the scientific functionality of the model, will be portable to various machines, and will not significantly degrade performance on existing supported platforms.
Biogeochemistry Working Group
One of the main accomplishments this year of the Biogeochemistry Working Group (BGCWG) has been the incorporation of active land, ocean, and atmosphere carbon cycle modules into the CCSM1 physical framework (CCSM1 carbon-climate model). The land biogeochemistry module is based on a merging of Carnegie-Ames-Stanford Approach (CASA) biogeochemistry and Land Surface Model (LSM) biogeophysics, with additional dynamic allocation and prognostic leaf phenology. The ocean module is based on a full-depth carbon, phosphorus, and oxygen model developed for the Ocean Carbon Model Intercomparison Project (OCMIP-2), with the addition of fully prognostic production and an active iron cycle.
Much of the work of the last year has focused on the integration of the carbon dynamics with the coupled model physics following a sequential spin-up strategy. Biases in the coupled CCSM physical solutions can introduce large drifts in land/ocean/atmosphere carbon inventories, and thus gradual adjustments are required before the full integration of the atmospheric CO2 with the physics through the radiation terms. Several multicentury spin-up runs (land-atmosphere and land-ocean-atmosphere) have been completed with the new land-ocean biogeochemistry modules and are under analysis.
Considerable effort also has been devoted within the BGCWG to the development of more sophisticated biogeochemistry components for the land and ocean within later versions of the CCSM. A fairly sophisticated marine ecosystem model has been implemented within an uncoupled version of the CCSM2 ocean physical model. The ecosystem model includes multiple element cycles (C, P, N, Si, Fe, O) and multiple plankton functional groups (picoplankton, diatoms, diazotrophs, calcifiers). Multidecade-long simulations have been conducted to explore the upper ocean behavior of the systems.
In conjunction with the LMWG, a new land biogeochemistry model is being developed within the CLM2 biogeophysical framework. The model explicitly includes nitrogen dynamics and has been extensively tested against data at individual sites, with preliminary work underway on regional and global model-data comparisons.
Work has been completed on a suite of past, present, and future atmospheric dust simulations within CCSM. Predictions of future dust levels are particularly important because they allow for the study of the impact of changing land surface processes on ocean biogeochemistry, as well as radiative feedbacks. The
results of the study suggest that “natural” aerosols have very strong responses to human interactions and should be more carefully studied for the climate and biogeochemical impact.
Polar Climate Working Group
Members of the Polar Climate Working Group (PCWG) have performed a number of simulations and analyses using CCSM2, including analysis of the 1000-year climate simulation and 1% per year increasing CO2 integrations. Others have continued model development and identifying areas for model improvements. Model analyses include the relation between high-latitude storm tracks and model biases, such as the position of the Siberian high and weak cyclogenesis near the Antarctic peninsula, variability of Antarctic sea ice and its interactions with the ocean and atmosphere, controls on the location of the sea ice edge, and investigations of polar amplification in CCSM2. Other simulations highlight the performance of CCSM components and parameterizations; for example, the single column model version of CCSM simulated variables at the Surface Heat Exchange Budget for the Arctic (SHEBA) site on scales of a few days to one year, with the exception of the cloud fraction. In response, members of the AMWG have made changes to the cloud physics parameterizations that improve polar simulations. Ongoing model sensitivity studies tested the effects of interactive sea ice and ocean model components within the CCSM framework, coupled model sensitivity to resolution of the ice thickness distribution, and sensitivity of the polar atmospheric circulation to horizontal resolution. Additional single column ice-ocean simulations of SHEBA conditions have been performed to investigate ice-ocean coupling issues and improved parameterizations of summertime lead conditions.
Several new features have been added to the sea ice model, including an incremental remapping advection algorithm that includes open water advection, non-zero sea ice reference salinity (with respect to ice-ocean exchanges), correction of wind and ice-ocean stress terms for the free drift regime, and a few minor bug fixes.
Ocean Model Working Group
The ocean component of CCSM has been upgraded in several ways. The absorption of solar radiation in the upper ocean is now governed by spatially and monthly dependent global fields of specified chlorophyll distributions derived from satellite ocean color observations. Compared to the previous constant absorption scheme, regions of high primary productivity have warmer SSTs and unproductive regions lower, even though the net solar surface radiation is unchanged. The numerics of the K-Profile Parameterization (KPP) vertical mixing scheme were improved to remove a shallow mixing bias. A more efficient barotropic solver has been implemented. A simple diurnal solar cycle was tested and is now a run time option, which results in a significant reduction in the cold bias of coupled solutions in the equatorial Pacific SST.
The very warm (>5°C) SST biases along the west coasts of South America, South Africa, and California have been investigated. These regions are physically similar in their abrupt near-coast orography, upwelling favorable equatorward coastal winds, and non-precipitating stratus clouds, which suggests a common cause for the biases. Numerical experiments indicate that these biases are more the cause of, rather than a passive response to, long standing model deficiencies in the Intertropical Convergence Zones (ITCZs). In a global coupled model experiment, near coastal ocean temperatures above 500m off South Africa and South America were forced to remain close to observations. There was a marked improvement in the ITCZ structures in hemispheres of both the Atlantic and Pacific. In the Atlantic, the SSTs both south and north of the equator are improved by as much as 5°C, and the precipitation simulations are also improved. In the Pacific the effects are felt as far away as New Guinea, where there is a significant increase in precipitation accompanied by a contraction of the spurious "double ITCZ" across the central and eastern South Pacific. There is also a marked improvement in the distribution of rainfall across the North Pacific ITCZ, with a bigger fraction falling in the east. These experiments demonstrate how local active regions can have long-range influences and the importance of getting good simulations of the processes in those areas.